The present invention relates generally to systems for recording information signals on recording mediums and systems for reproducing the recorded signals from the recording mediums. More particularly, the invention relates to a system for recording a main information signal on a rotary recording medium. This main signal includes a video signal and an audio signal together with reference signals the frequencies of which are selected so that beat disturbance will not occur. The invention also relates to systems for reproducing these signals from the recording medium.
Known systems heretofore recorded reproduced information signals such as video signals and/or audio signals on and from disc-shaped, rotary mediums (referred to hereinafter simply as "discs"). These known systems are broadly divided into systems in which reproduction is carried out optically, systems in which reproduction is carried out by means of a reproducing stylus employing a piezoelectric element, and systems in which reproduction is carried out by utilizing variations in the electrostatic capacitance between an electrode on a reproducing stylus and the recorded surface of the disc.
We have considered the advantages and disadvantages of these three kinds of systems and have adopted the electrostatic capacitance system as being the most desirable. Further, we have previously described a new system for overcoming the problems accompanying known electrostatic capacitance system. Please see copending application entitled "Information signal recording system", U.S. patent application Ser. No. 785,095, filed Apr. 6, 1977.
More specifically, in accordance with a known electrostatic capacitance system, recording is accomplished by forming a spiral guide groove in the disc for guiding the reproducing stylus. At the same time, pits are formed responsive to and in correspondence with an information signal to be recorded on the bottom surface of the groove. In the reproducing system, the reproducing stylus, is guided by the guide groove. It traces the track within the groove and reproduces the recorded signal in response to variations in the electrostatic capacitance.
In this known system however, a reproducing stylus guiding groove is provided in the disc. The reproducing stylus is compulsorily guided by this groove. It is not possible for the reproducing stylus to do such things as riding over the groove wall of one track, moving into another track, and then returning to the original track. If the reproducing stylus were to be forced to undergo such an operation, the groove and the stylus would be damaged. For this reason, it has not been possible in this known system to carry out quick-motion picture reproduction, slow-motion picture reproduction, still-picture reproduction, and the like.
Futhermore, since the contact area between the tip of the reproducing stylus and the guide groove of the disc is small, the stylus wears excessively. When the width of the stylus tip reaches the groove width as a result of abrasive wear, the serviceable life of this reproducing stylus ends. Thus, this system is accompanied by the problem of a short life for the reproducing stylus. Another problem is that fine chip particles are abraded from the disc by the tracing action of the reproducing stylus along the guide groove. These chips give rise to difficulties which further promote wear and damage of the stylus and the guide groove. As a result, there is an increase of the gap between the electrode at the stylus tip and the recorded surface including the pits within the disc guide groove. The reproduced output decreases because of spacing loss.
If a low rotationsl disc speed (such as 450 rpm.) is used value such as 450 rpm., for example, in order to prolong the life of the stylus tip, the recording wavelength of the information signal recorded in the guide groove will become short. For this reason, a low price recording apparatus using a laser light cannot be employed in the recording system. The disadvantageous result is that a high price recording apparatus must use an electron beam.
Accordingly, to overcome the various problems of the known systems described above, the aforecited patent application describes a novel system for recording and reproducing information signals. According to the recording in this system, pits are formed in accordance with the information signal being recorded along a spiral track on a recording medium of flat disc shape, without forming a groove therein. In the reproducing system, a reproducing stylus traces over and along this track to reproduce the recorded information signal. In this system, pilot or reference signals are recorded on or in the vicinity of a track of the information signal such as video signal on a rotary disc. At the time of reproducing, the reference signals are reproduced together with the video signal. Tracking servo control is carried out so that the reproducing stylus traces accurately along the track in response to the reproduced reference signals.
By the use of this previous system, the recording track has no groove. There is no possibility whatsoever of the reproducing stylus or the recording medium being damaged. The stylus can trace the same portion of the track repeatedly many times. Therefore, special reproduction such as still, slow motion, or quick motion reproduction becomes possible. Furthermore, other difficulties of the known system are removed.
If a video signal and an audio signal are to be recorded as the main information signal, it is desirable to record the video signal and the audio signal on the same track for effective utilization of the recording capacity of the recording medium. Accordingly, known recording systems use a duty-cycle modulation system. A frequency-modulated audio signal obtained by frequency modulating a carrier wave with an audio signal is superimposed on a frequency-modulated video signal obtained by frequency modulating a carrier wave with a video signal. The duty cycle of the carrier wave of the frequency-modulated video signal is modulated by the carrier wave of the frequency-modulated audio signal.
In this system, however, distortion in the signal transmission system is great. Cross-modulation distortion arises between the frequency-modulated video signal and the frequency-modulated audio signal. As a result, beat disturbance occurs in the demodulated video signal. For reducing this beat disturbance, the level of the frequency-modulated audio signal is lowered relative to the frequency-modulated video signal. However, this is not desirable since it gives rise to a lowering of the S/N ratio of the demodulated audio signal.
Furthermore, if a color video signal and an audio signal are recorded on the same track, one conceivable method begins by separating the color video signal into a luminance signal and a chrominance signal. The frequency-modulated audio signal and the separated luminance signal are mixed. Then a carrier wave is frequency modulated with the resulting mixed signal. The system mixes and records the resulting frequency-modulated signal. A signal is obtained by frequency converting the separated chrominance signal to a band which is lower than the frequency-modulated luminance signal. By this method, however, the chrominance signal which has been frequency converted to a lower frequency band is not frequency modulated. This chrominance signal cannot be passed through an amplitude limiter in the reproducing system. There is a problem of its readily being affected by level fluctuation.
On the other hand, if pilot signals are recorded in correspondence with the horizontal blanking periods of the video signal in the aforedescribed system, the pilot signals have no disturbing effect whatsoever on the video signal. However, the pilot signals may be used for tracking control of the reproducing transducer and also for correction control of time-axis error (jitter) of the reproduced signal. The pilot signals may also be used for arm-stretcher control by which the position of a reproducing transducer is controlled in its longitudinal direction along the track. The pilot signals are recorded in correspondence with all periods of the video signal. In this case, it is conceivable that the pilot signals may impart a beat disturbance to the video signal. Therefore, there is a necessity for preventing deleterious effects on the video signal even when the pilot signals are recorded over all periods of the video signal.